> ICs to sample 100 lines are not too costly, but you MAY be able to use
> an addressed =A0XY or even XYZ matrix to use many less active devices,
> or to use many diodes and fewer ICs.
>
> Using XY you need 10 x 10, or in multiples of 4 say =3D 8x12 =3D 96.
>
> If you can work an XYZ addressing scheme you can get say 5x5x5 =3D 125
> or 4x4x8 =3D 128 .
>
> In an XY system an X line drives a row (say) of inputs with matrix
> points being held down (or up) by input voltages. The y colums read
> the result. This may end up with more R's and / o r diodes than are
> worthwhile.

Some quick doodling suggests that you MAY be able to produce a
multiplexed array with 3 x diode and 1 x resistor per sample point,
plus matrix addressing logic. Probably better going with an IC
solution, but this could be workable at small size in production
volumes. For a one-off or small run then ICs much better.

I and several others mentioned serial shift registers and 4051 style
transmission gates were mentioned.

Using 8:1 addressable selectors (eg CD4051) you can address an array
of points randomly with a parallel address. ie whereas with serial
shift register schemes you have to clock the whole array (or an Nth
part in some arrangements) of data in to read a single point. With a
fully addressable array of switches you can concentrate on a selected
few inputs if desired.

Using CD4051s.
12 4051 x 8 bit =3D 96 inputs.
Address 12 4051's requires 4 bits of address (up to 16 can be addressed).

Using parallel input to PIC you need 8 data lines and 4 address lines
=3D 12 pins total.
OR you could serial clock the data in g\from the 405a array with an eg
CD3\4021 to get a hybrid and fewer PIC  pins.

Or ... - many possible answers.

If you can stand having to clock the array 96 times per array read
it's hard to beat the CD4021's on price.

4021 x 8 bit so 12 needed.
Digikey $US0.40 x 12 =3D $4.80.
3 ? PIC pins to drive.

The really really really enthused can do this with ONE PIC pin using
it for clocking, reset, and data read - with a little 'glue' . This
level of price saving  not usually required.


 Russell McMahon

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